Your Swiss Half Fare Card should have blank fields named something similar to "Pass/ID-Nr", "Name", "Signature" (and perhaps "Country") on the right side. This is where you should write down your passport number, your name as stated on your passport, and your signature (and country of residence). Now your pass is ready to use. I suppose a start and end date is already printed on the pass, right?

If you have a German version: Signature = Unterschrift and Country = Land.

Don't forget to take along your passport while traveling. Conductors may want to check it.